Iron carbide on titania surface modified with group VA oxides as Fisher-Tropsch catalysts

ABSTRACT

Catalysts comprising iron carbide on a surface modified titania support wherein said support comprises a surface modifying oxide of tantalum, niobium, vanadium and mixtures thereof supported on said titania wherein at least a portion of said surface modifying oxide is in a non-crystalline form. These catalysts are useful for Fischer-Tropsch hydrocarbon synthesis reactions. Preferably, at least about 25 wt. % of said surface modifying oxide will be in a non-crystalline form.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to catalyst compositions of matter comprisingiron carbide supported on a surface modified titania support. Moreparticularly, this invention relates to Fischer-Tropsch catalystcompositions comprising iron carbide supported on a surface modifiedtitania support, wherein said support comprises a surface modifyingoxide of tantalum, vanadium, niobium or mixture thereof supported on thesurface of said titania and wherein at least a portion of said surfacemodifying oxide is in a non-crystalline form.

BACKGROUND OF THE DISCLOSURE

The use of iron-titania mixtures as Fischer-Tropsch catalysts forconverting mixtures of CO and H₂ to hydrocarbons is well-known to thoseskilled in the art. For example, U.S. Pat. No. 2,543,327 disclosestitania promoted iron oxide for Fischer-Tropsch synthesis wherein theiron oxide is in the form of naturally occurring magnetite andpreferably as Alan Wood ore. In this disclosure a typical catalyst isshown as prepared by mixing about 13,600 grams of Alan Wood ore with 98grams of titania and 216 grams of potassium carbonate used as apromoter. The ratio of hydrogen to carbon monoxide disclosed as beingpreferably at least 2/1 and the results show that the catalyst hasrelatively poor activity with a large selectivity towards the productionof methane and very little selectivity towards the production of C₂ +hydrocarbons. That is, the Fischer-Tropsch product was primarilymethane. Similarly, British Pat. No. 1,512,743 also discloses a titaniapromoted, massive iron type of Fischer-Tropsch catalyst wherein ironoxide is mixed with titanium oxide, zinc oxide and potassium carbonatewith the resulting mixture being sintered and then reduced for manyhours at 500° C. Although this catalyst has relatively reasonablyactivity with regard to conversion of the CO and H₂ mixture, the productwas primarily (i.e., about 73%) olefinic, unsaturated C₂ /C₄hydrocarbons and with only about 10% of C₂ /C₄ saturated hydrocarbons oralkanes being produced. U.S. Pat. Nos. 4,192,777 and 4,154,751 whiledirected towards the use of potassium promoted Group VA metal clustercatalysts in Fischer-Tropsch synthesis reactions, suggest that ironsupported on titania would be useful Fischer-Tropsch catalysts but donot disclose the preparation of same. In their examples, they discloseiron on various supports other than titania with the amount of iron onthe support generally being less than about 5 percent. U.S. Pat. No.4,261,865 discloses an iron titanate-alkali metal hydroxide catalyst forpreparing alpha-olefins for mixtures of CO and H₂. That is, the catalystis not iron supported on titania along with an alkali metal hydroxidebut rather an iron titanate compound.

Another example of a titania-promoted massive iron catalyst forFischer-Tropsch synthesis may be found in the Volume 17, No. 3-4 React.Kinet. Catal. Lett., pages 373-378, (1971) titled "Hydrocondensation ofCO₂ (CO) Over Supported Iron Catalysts". This article discloses an ironoxide, titania, alumina, copper oxide catalyst promoted with potasium.Similarly, in European patent application EP 0 071770 A2 Fischer-Tropschcatalysts are disclosed which include iron-titania catalysts wherein theiron to titania ratio can be greater than 1/10. The actual iron-titaniacatalyst is not an iron supported on titania catalyst but aniron/titania catalyst produced by a coprecipitation technique whereinthe active iron catalytic component is distributed throughout a titaniumoxide matrix. Thus, the resulting catalyst was not iron supported ontitania but rather a bulk phase iron/titania mixture which, when usedfor Fischer-Tropsch synthesis, produced predominantly olefins. Theamount of olefins produced was generally greater than about 80% of thetotal hydrocarbon product.

With regard to iron/titania catalysts for Fischer-Tropsch wherein theiron is supported on titania, a 1982 article by Vannice,Titania-Supported Metals as CO Hydrogenation Catalysts, J. Catalysis, v.74, p.199-202 (1982), discloses the use of an iron/titania catalyst forFischer-Tropsch synthesis wherein the amount of iron, calculated asmetallic iron, is 5 percent of the iron/titania composite and thecatalyst shows extremely little activity for Fischer-Tropsch synthesis.An article by Reymond et al, Influence of The Support or of an Additiveon The Catalytic Activity in The Hydrocondensation of Carbon Monoxide byIron Catalysts in "Metal-Support and Metal-Additive Effects inCatalysis", B. Imelik et al (Eds), Elsevier, Netherlands, p.337-348(1982), also discloses the use of iron/titania Fischer-Tropsch catalystswherein the iron is supported on the titania.

U.S. Pat. No. 4,149,998 to Tauster et al. relates to heterogeneouscatalysts consisting of Group VIII metals, including iron, dispersed onoxide carriers selected from the group consisting of Ti, V, Nb, Ta andmixtures thereof and zirconium titanate and BaTiO₃. However, there is nosuggestion in this patent that the catalytic metal be dispersed on asurface modified titania.

SUMMARY OF THE INVENTION

It has now been discovered that catalysts comprising iron carbidesupported on a surface modified titania support wherein said supportcomprises a surface modifying oxide of tantalum, niobium, vanadium andmixture thereof supported on the surface of said titania and wherein atleast a portion of said surface modifying oxide is in a non-crystallineform are useful catalysts for Fischer-Tropsch hydrocarbon synthesis.Moreover, Fischer-Tropsch reactions conducted with these catalysts havebeen found to result in increased olefin and decreased methane makecompared to Fischer-Tropsch catalysts comprising iron supported ontitania wherein the surface of the titania has not been modified with aGroup VA modifying oxide. Further, the catalysts of this inventionproduce a greater amount of heavier products and exhibit superiorcatalyst maintenance than similar catalysts on titania whose surface hasnot been modified with a Group VA oxide.

In a preferred embodiment at least about 25 wt. % of the surfacemodifying oxide of tantalum, niobium, vanadium or mixture thereofpresent on the titania surface will be in a non-crystalline form. In aparticularly preferred embodiment, the catalyst will be pretreated withCO at elevated temperature prior to use.

DETAILED DESCRIPTION

The term surface modified titania as used herein refers to titania whosesurface has been modified by an oxide of niobium, vanadium, tantalum andmixture thereof in an amount such that the modified support exhibitsproperties different from titania whose surface has not been modifiedand also different from bulk niobia, tantala, vanadia and mixturethereof. Concomitantly, the final catalyst composition will exhibitproperties different from iron carbide supported on unmodified titaniaor on bulk niobia, tantala, vanadia and mixture thereof.

Thus, the catalyst support useful for preparing the catalysts of thisinvention comprise titania whose surface has been modified with an oxideof a Group VA metal (vanadium, niobium, tantalum and mixture thereof).That is, the surface of the titania has been modified by an oxide ofvanadium, niobium, tantalum and mixture thereof in an amount such thatthe catalyst exhibits properties different from titania whose surfacehas not been modified and different from bulk oxides of vanadium,niobium, tantalum and mixture thereof. Those skilled in the art knowthat the oxides of niobium, tantalum, vanadium and mixtures thereof arecrystalline in their bulk form. Thus, at least a portion of andpreferably at least about 25 wt. % of the Group VA metal oxide will bein a non-crystalline form. This will be accomplished if the metal oxideloading on the titania broadly ranges between about 0.5 to 25 wt. % ofthe total catalyst weight.

In the catalyts of this invention the iron carbide is supported on thesurface modified titania. Consequently, the catalysts of this inventionare prepared by a two-step sequential process wherein the surfacemodified titania support is prepared first, followed by depositing theiron carbide or iron carbide precursor on the support. Thus, in thefirst step an oxide or precursor thereof of a metal selected from thegroup consisting of niobium, tantalum, vanadium and mixture thereof isdeposited on the titania to form either the surface modified support or,in the case of one or more precursors, a support precursor. The supportprecursor will then be calcined to oxidize the oxide precursor and forma support comprising titania whose surface has been modified by an oxideof a metal selected from the group consisting of niobium, tantalum,vanadium and mixture thereof wherein at least a portion of said surfacemodifying oxide is in a non-crystalline form.

The catalyst support precursors of this invention may be prepared bytechniques well-known in the art, such as incipient wetness,impregnation, etc., the choice being left to the practitioner. Whenusing the impregnation technique, the impregnating solution is contactedwith the titania for a time sufficient to deposit the oxide precursormaterial onto the titania either by selective adsorption oralternatively, the excess solvent may be evaporated during dryingleaving behind the precursor salt. If an impregnation or incipientwetness technique is used to prepare a support precursor of thisinvention, the transition metal oxide salt solution used may be aqueousor organic, the only requirement being that an adequate amount ofprecursor compound for the selected Group VA transition metal oxide oroxides be soluble in the solvent used in preparing this solution.

The support precursor composite will then normally be dried attemperatures ranging from about 50°-300° C. to remove the excess solventand, if necessary, decompose the salt if it is an organic salt to form acatalyst precursor. The support precursor composite is then convertedinto the surface modified titania support by calcining at temperaturesof from about 150° to 800° C. and preferably 300°-700° C. in a suitableoxidizing atmosphere such as air, oxygen, etc. The time required tocalcine the composite will, of course, depend on the temperature and ingeneral will range from about 0.5-7 hours. Reducing atmospheres may alsobe used to decompose the transition metal oxide precursors, but theresulting composite will then require subsequent calcination to convertthe reduced metal component to the oxide form.

The supports of this invention will generally have metal oxide loadingsof from about 0.5 to 25 wt. % metal oxide on the titania based on thetotal support composition, preferably from about 1 to 15 wt. %, morepreferably from about 2-10 wt. % based on the total support composition.

It is important to this invention that the iron carbide is supported onand not merely mixed with the surface modified titania support.

The catalyst will be prepared by depositing a suitable iron precursorcomponent onto the surface modified titania support from a precursorsolution using any of the well-known techniques such as incipientwetness, multiple impregnation, pore-filling etc., the choice being leftto the convenience of the practitioner. As has heretofore been stated,it is important for the iron precursor to be deposited onto the supportas opposed to other methods for catalyst preparation such asco-precipitation or physical mixtures. After impregnation, theimpregnate is dried to remove excess solvent and/or water therefrom. Thedry impregnate can then be converted to a catalyst of this inventionemploying a number of different methods. In one method, the impregnatewill be converted directly to a catalyst of this invention by contactingsame with a CO containing reducing gas, preferably a reducing gascontaining a mixture of CO and H₂. Thus, it will be appreciated to thoseskilled in the art that the catalyst of this invention can be formedfrom the impregnate in-situ in a Fischer-Tropsch hydrocarbon synthesisreactor. However, it is preferred to employ a sequential treatment offirst contacting the dry impregnate with an H₂ containing reducing gasthat does not contain CO to reduce the impregnate, followed bycontacting the reduced impregnate with CO or a CO containing gas such asa mixture of CO and H₂ to form the catalyst of this invention. As apractical matter, it may be commercially advantageous to form thecatalyst of this invention by subjecting the impregnate to calcining toconvert the supported iron precursor component to iron oxide, followedby subsequent reduction and formation of the catalyst of this invention.

Promoter metals such as potassium or other alkali metals may be addedvia impregnation, etc. before the composite is contacted with a reducingatmosphere and/or CO containing gas to form the catalyst of thisinvention. In general, the amount of promoter metal present will rangefrom about 0.5 to 5 wt. % based on the amount of iron (calculated as Fe₂O₃) supported on the titania.

If one desires to obtain a catalyst of this invention via a supportediron oxide route, then the dry impregnate will be calcined in air orother suitable oxidizing atmosphere at a temperature of from about 120°to 300° C. for a time sufficient to convert the supported iron precursorcomponent to iron oxide. After the iron/surface modified titaniaimpregnate has been calcined to convert the supported iron precursorcompound to iron oxide, the iron oxide/titania composite, with orwithout one or more promoter metals, is reduced in ahydrogen-containing, net-reducing atmosphere at a temperature broadlyranging from about 300°-500° C. for a time sufficient to convert theiron oxide to metallic iron. It has been found that if one tries toreduce the iron oxide/titania composite at a temperature below about300° C., (i.e., 250° C.), the catalyst of this invention will notsubsequently be formed.

Irrespective of the route one employs to form a catalyst of thisinvention, whether by reduction followed by contacting with CO, directformation of the catalyst or through the supported iron oxide route, itis important not to contact the composite with a reducing gas attemperatures above about 500° C.

Reduction temperatures exceeding about 500° C. will produce a catalystwhich exhibits relatively low CO hydrogenation activity with less than50% of the C₂ +hydrocarbons being alkanes. Further, even at a 500° C.reduction temperature a less effective catalyst will be produced if thereduction occurs for too long a time, i.e., about ten hours or more.Thus it will be appreciated that the temperature range for reducing thecomposite to form a catalyst cannot be critically quantified with anydegree of precision inasmuch as there exists a time-temperaturecontinuum for proper reduction.

In a preferred embodiment of this invention, the catalyst composite willfirst be reduced, followed by contacting with CO at temperatures rangingfrom about 200° to 500° C. and preferably 200° to 400° C. for a timesufficient to form a catalyst comprising iron carbide supported on thesurface modified titania. It has been found that a CO treatmentfollowing hydrogen reduction dramatically improves the activity of thecatalyst for CO conversion with only slight changes in productselectivity. Iron carbide on the surface modified titania support willalso be achieved by treating the calcined iron/support composite with amixture of CO and H₂, but it is preferred to use the sequentialtreatment comprising hydrogen reduction followed by CO treatment.Further, when using this sequential treatment to produce a catalyst ofthis invention, it is preferred that the temperature used for the COtreatment be lower than that used for the hydrogen reduction. Thus, ingeneral the CO treatment will occur at a temperature of about 100° to200° C. lower than the temperature used for the hydrogen reduction.

It has also been discovered that, if a catalyst of this invention hasbeen prepared by hydrogen reduction and then contacted in-situ, in areactor, with a feedstream comprising a mixture of CO and H₂ to form acatalyst of this invention, the activity of the so formed catalyst willbe substantially increased by reducing or eliminating the hydrogencontent of the feedstream, raising the temperature in the reactor anadditional 50° to 150° C. for a short period of time (i.e., 3-5 hours),followed by reestablishing the original reaction conditions.

Predominantly C₂ + alkane hydrocarbons are produced from mixtures of COand H₂ by contacting said mixtures with the catalyst of this inventionat temperatures ranging from about 200° to 350° C. and preferably fromabout 250°-320° C. The reaction pressure will generally range from about100-500 psig and more preferably from about 150-300 psig, althoughpressures outside this range may be used if desired. However, if onegoes too low in pressure (i.e., <50 psig), catalyst activity will begreatly reduced and methane production will predominate. Upper pressurelimits will generally be dictated by economic considerations. The H₂ /COmole ratio in the reaction zone will generally range from about 1/2 to3/1, preferably from about 1/2 to 2/1 and still more preferably fromabout 1/2 to 1/1.

The invention will be more readily understood by reference to thefollowing examples.

CATALYST SUPPORT PREPARATION

Degussa P-25, a mixture of anatase and rutile titania, was used as thetitania support. Both of the catalyst supports were prepared in a glovebox in a nitrogen atmosphere to prevent decomposition of the transitionmetal oxide precursors. In all cases 10 grams of the P-25 titania powderwere slurried in 100 cc of ethanol to which was added the transitionmetal oxide precursor, with the resulting mixture stirred overnight,under flowing nitrogen, to evaporate the ethanol. Each dry mixture wasthen taken out of the glove box and 3 cc of water added. The resultingmixture was stirred overnight in air, then the dry powder placed in aquartz boat and slowly heated in a 1/1 flowing mixture of O₂ in He up to400° C. At 400° C. the He flow was cut off and the powdered, catalystsupport precursor then heated from 400° to 575° C. in 100% O₂. Eachsample of catalyst precursor was held at 575° C. in the O.sub. 2 for twohours to calcine the precursor into a surface modified titania supportof this invention.

The transition metal oxide precursors were obtained from Alfa, Inc. andwere Nb(C₂ H₅ O)₅ and VO (C₂ H₅ O)₃. The amounts of niobia and vanadiaprecursors added to each slurry of 10 g P-25 in 100 cc of ethanol were2.5 and 0.46 grams, respectively. The resulting catalysts contained 10wt. % niobia on titania and 2 wt. % vanadia on titania. The niobia andvanadia contents of the catalysts were expressed as niobium pentoxideand vanadium pentoxide.

EXAMPLE 1

A 5-8 cc sample of catalyst, containing 4 wt. % Fe as elemental iron onthe support, was loaded into a 3/8 inch O.D. 316 stainless steel tubularreactor. The system was then flushed with nitrogen at atmosphericpressure and then flushed with 90% H₂ /10% N₂ at atmospheric pressure.The reactor was then heated to 500° C. in flowing 90% H₂ /10% N₂ (100cc/min) and maintained at these conditions for 5 hrs. After this, thereactor was cooled to the desired reaction temperature, 290°-315° C.,and the pressure increased to 300 psig. The reducing gas was thenreplaced with 1/1 H₂ /CO at a flow rate (standard hourly space velocity)of 500 V/V/hr. The exit gas from the reactor was fed into a gaschromatograph for on-line analysis of C₁ -C₁₅ hydrocarbons, CO, CO₂ andN₂.

The results of Runs. A-D are presented in Table 1. The runs can becompared either at conditions for nearly equal conversion, run A vs. RunC and Run B vs. Run D, or at identical conditions, Run A vs. Run D. Inall cases the vanadium containing system is found to generate lowerlevels of methane than the standard catalyst. Comparison of thesecatalysts at identical conditions, Run A vs. Run D at 350° C., alsoindicates that the vanadium containing system is more active. Themodified TiO₂ catalyst of the present invention is clearly superior tothe unmodified analog for production of desired α-olefin products whileminimizing the formation of unwanted methane.

                  TABLE 1                                                         ______________________________________                                                              4 wt. % Iron on                                                               Titania Surface                                                   4 wt. % Iron                                                                              Modified With An                                                  on Titania  Oxide of Vanadium                                       Run         A        B        C      D                                        ______________________________________                                        Temp. °C.                                                                          305      315      290    305                                      % CO Conversion                                                                           47       60       49     70                                       Wt. % Selectivity                                                             (CO.sub.2 Free)                                                               CH.sub.4    21.0     24.4     16.1   17.8                                     C.sub.2.sup.=                                                                             0.4      0.6      2.7    2.8                                      C.sub.2 °                                                                          16.1     16.2     20.2   16.8                                     C.sub.3.sup.=                                                                             11.7     9.7      16.6   18.6                                     C.sub.3 °                                                                          13.7     12.7     14.4   16.5                                     C.sub.4.sup.=                                                                             2.5      3.0      1.9    2.0                                      C.sub.4 °                                                                          7.1      7.4      8.1    10.9                                     C.sub.5.sup.+                                                                             27.5     26.0     20.0   14.6                                     ______________________________________                                         Conditions: 1:1 H.sub.2 :CO, 500 v/v/hr, 300 psig, pretreatment with          H.sub.2 at 500° C. for 5 hr. C.sub.5.sup.+  determined by nitrogen     internal standard method.                                                

The activity and carbon number distributions for the unmodified Fe/TiO₂and the V and Nb surface modified Fe/TiO₂ catalysts duringFischer-Tropsch synthesis are presented in Table 2. The addition of Vand Nb affected the activity and selectivity of the Fe/TiO₂ catalysts.The incorporation of V and Nb to the TiO₂ surface increased anddecreased the conversion of CO, respectively. The stability of themodified catalysts was superior to that of the unmodified Fe/TiO₂catalyst (less coking). The V and Nb modified Fe/TiO₂ catalystssubstantially decreased the CH₄ yield and increased the C₅ + yield.Whereas Fe/TiO₂ yields substantial amounts of paraffins (70-90%)paraffins in hydrocarbon) the V and Nb modified Fe/TiO₂ catalystsproduced substantial amounts of olefins and low amounts of paraffins. Inaddition, XRD analysis of the spent V and Nb modified Fe/TiO₂ catalystsdid not show the presence of FeTiO₃ in the catalysts. Thus, the additionof V and Nb to the surface of the TiO₂ altered the Fe--TiO₂ interactionand the nature of the products obtained from such a catalyst duringFischer-Tropsch synthesis.

                  TABLE 2                                                         ______________________________________                                                  4%     4% Fe(TiO.sub.2 +                                                                         4% Fe(TiO.sub.2 +                                          Fe/TiO.sub.2                                                                         V oxide)    Nb oxide)                                        ______________________________________                                        % CO Conversion                                                                           27.0     34.3        20.1                                         C.sub.1     21.0     13.0        14.6                                         C.sub.2     17.0     21.5        15.0                                         C.sub.3     29.0     12.0        11.9                                         C.sub.4     9.0      8.0         6.0                                          .sup. C.sub.5.sup.+                                                                       24.0     45.5        52.5                                         ______________________________________                                         CONDITIONS: 270° C., 300 psia, 500-600 V/V/M, H.sub.2 :CO = 1     

What is claimed is:
 1. A process for producing hydrocarbons, includingalkane hydrocarbons, from gaseous feed mixtures of CO and H₂ comprisingcontacting said feed, at a temperature ranging from about 200° to 350°C. and for a time sufficient to convert at least a portion of said feedto alkane hydrocarbons, with a catalyst comprising iron carbidesupported on a surface modified titania support wherein said supportcomprises an oxide of a metal selected from the group consisting ofniobium, vanadium, tantalum and mixture thereof supported on titaniawherein at least a portion of said supported oxide is in anon-crystalline form.
 2. The process of claim 1 wherein said catalystcontains one or more alkali promoters.
 3. The process of claim 2 whereinthe amount of iron carbide, calculated as iron, ranges from about 2 to20 wt. % of the total catalyst composition.
 4. The process of claim 3wherein at least about 25 wt. % of said supported oxide is innon-crystalline form.
 5. The process of claim 4 wherein the amount ofsupported iron carbide, calculated as iron, ranges from about 4 to 10wt. % of the total catalyst composition.
 6. A process for producinghydrocarbons, including alkane hydrocarbons, from gaseous feed mixturesof CO and H₂ comprising contacting said feed, a temperature ranging fromabout 200°-350° C. and for a time sufficient to convert at least aportion of said feed to alkane hydrocarbons, with a catalyst comprisingiron carbide supported on a surface modified titania support whereinsaid support comprises an oxide of a metal selected from a groupconsisting of niobium, vanadium, tantalum and mixture thereof supportedon titania wherein at least a portion of said supported oxide is in anon-crystalline form, said catalyst having been formed by a processcomprising the steps of:(a) depositing an iron precursor compound on thesurface modified titania support in an amount such that the finalcatalyst will contain supported iron in an amount of at least about 2milligrams of iron, calculated as Fe₂ O₃, per square meter of supportsurface; (b) calcining the iron precursor supported on the surfacemodified titania support produced in step (a) for a time sufficient todecompose said iron precursor material and convert at least a portion ofsaid supported iron to Fe₂ O₃ and form a calcined composite; (c)contacting said calcined composite formed in step (b) with hydrogen atelevated temperature for a time sufficient to convert at least a portionof said supported iron to a reduced composite; and (d) contacting saidreduced composite formed in (c) with CO at elevated temperature of atleast about 200° C. for a time sufficient to form said catalyst.
 7. Theprocess of claim 6 wherein said reduced composite is contacted with COat a temperature broadly ranging from 200°-500° C. prior to use.
 8. Theprocess of either of claims 6 or 7, wherein said catalyst contains oneor more alkali metal promoters.
 9. The process of claim 8 wherein theamount of iron carbide present in said catalyst, calculated as iron,ranges from about 2 to 20 wt. % of the total catalyst composition. 10.The process of claim 9 wherein at least about 25 wt. % of said supportedoxide of niobium, tantalum, vanadium or mixtures thereof isnon-crystalline.
 11. The process of claim 10 wherein the amount ofsupported iron carbide, calculated as iron, ranges from about 4 to 10wt. % of the total catalyst composition.